1. Field of the Invention
This application concerns an oscillator circuit. In particular, when a direct current (DC) voltage is applied, the circuit can provide an output voltage that is greater than the applied DC voltage.
2. Description of Related Art
Oscillators convert a DC supply voltage, such as a battery, into an alternating current (AC) output. There are various types of oscillator circuits. Depending on the field of application, an oscillator may be required to have certain operational parameters, such as low power consumption, low phase noise, high oscillation frequency, wide oscillation frequency adjustment range, low sensitivity to interference signals and/or low manufacturing cost.
By way of example, some LC oscillators, which include frequency determining components such as inductors (L) and capacitors (C), achieve a number of the above-listed parameters. The values of the inductors and capacitors determine the frequency (oscillation frequency) of the resultant AC output.
FIG. 1 illustrates a basic LC Oscillator Tank Circuit. The circuit consists of an inductive coil, L′ (L prime) and a capacitor, C′ (C prime). The capacitor stores energy in the form of an electrostatic field (voltage) across its plates, a and b, while the inductive coil L′ stores energy in the form of an electromagnetic field. When switch S1 is in position 1, the capacitor C′ charges up to the DC supply voltage of battery B. The switch is then moved to position 2, which puts the capacitor C′ in parallel with inductor L′. Capacitor C′ will discharge through inductor L′, resulting in the voltage across C falling as the current through L begins to rise in a first direction. The increase in current in L′ causes electromagnetic field around L′, which resists this flow of current. When capacitor C′ is completely discharged the energy that was originally stored in capacitor C′ as is now stored in the inductive coil L′ as an electromagnetic field around the coil windings.
Because there is no external voltage in the circuit to maintain the current within the coil, the electromagnetic field begins to collapse. The collapse of the magnetic field causes a back electromotive force (back EMF) to be induced in inductor L′, which attempts to keep the current flowing in the original direction. This current now charges up capacitor C′ with the opposite polarity to its original charge. C′ continues to charge up until the current reduces to zero and the electromagnetic field of the coil has collapsed completely. The energy originally introduced into the circuit through the switch, has been returned to the capacitor which again has a voltage potential across it, although it is now of the opposite polarity. The capacitor now starts to discharge again back through the coil L′ and the whole process is repeated. The polarity of the voltage changes as the energy is passed back and forth between the capacitor and inductor. This process would repeat indefinitely but for energy losses in the transfer between L′ and C′. Accordingly, circuitry has been developed to replace the lost energy.
FIG. 2 illustrates a Transistor LC Oscillator is one circuitry that uses a transistor as an amplifier switch to take a part of the output from the LC Tank circuit, amplify it and feed the energy back into the LC tank circuit.
A transistor Q′ (Q prime) is used as the LC oscillator amplifier. LC tank circuit 200 is the collector load of Q′. The amplifier 202 includes transistor Q and another coil L″ (L double prime) that has an electromagnetic field mutually coupled to the field of L′ and is connected between the base and the emitter of transistor Q. Accordingly, mutual inductance exists between tank circuit 200 and amplifier circuit 202. The changing current flowing in one coil circuit induces, by electromagnetic induction, a potential voltage in the other. As such, when the oscillations occur in LC tank circuit 200, electromagnetic energy is transferred from coil L′ to coil L″ and a voltage of the same frequency as that in tank circuit 200 is applied between the base and emitter of transistor Q. In this way the necessary automatic feedback voltage is applied to the amplifying transistor.
The amount of feedback is controlled by the coupling between the two coils L′ and L″. In order to maintain oscillations, the voltage applied to the tank circuit must be “in-phase” with the oscillations occurring in the tank circuit. This is achieved by winding the coil of L″ in the correct direction relative to coil L′ giving the correct amplitude and phase relationships for the oscillators circuit. The output voltage Vout is sinusoidal and such oscillators are often referred to as “harmonic oscillators.”
Harmonic oscillators come in many different forms because there are many different ways to construct an LC tank circuit and amplifier with the most common being the Hartley LC Oscillator, Colpitts LC Oscillator, Armstrong Oscillator and Clapp Oscillator to name a few.
In FIG. 3, Integrated circuit timer 555, R1′ (R1 prime) and C1′ (C1 prime) comprise an oscillating unit. The oscillating frequency varies with the value of R1′ and C1′. With R1′=12 KΩ and C1′=2200 pF, the oscillating frequency will be about 20 kHz and the output dc voltage Vout will be about 2.2 times as much as the voltage of power supply Vin. The output current could reach as much as 50 mA. Bipolar transistors are used in this circuit. In this circuit the voltage of the power supply has to be equal to or greater than about 4.5V to operate the 555 timer.
Circuitry may be added to oscillator circuits to boost the output voltage as compared to the input voltage. FIG. 4 illustrates an LC oscillator having two stages of amplification realized by VT1 and VT2. Vin can be 1.5V and the amplification result in Vout=9V. In this case, VT3 and VD3 act as a series pass regulator to keep Vout from rising above the breakdown voltage of VD3 and base-emitter of VT3.